Add __all__ part 2 (#13719)

---------

Co-authored-by: Avasam <samuel.06@hotmail.com>

stubs/greenlet/@tests/stubtest_allowlist.txt

@@ -1,6 +1,3 @@
-# TODO: missing from stub
-greenlet.__all__
-
 # Error: is not present in stub
 # =============================
 # this module only contains C code and exports no Python code, so it's better

stubs/greenlet/greenlet/__init__.pyi

@@ -11,3 +11,4 @@ from ._greenlet import (
 )
 
 __version__: Final[str]
+__all__ = ["__version__", "_C_API", "GreenletExit", "error", "getcurrent", "greenlet", "gettrace", "settrace"]